Electronic switching power supplies have all but replaced the previously conventional type of power supply using series regulation in the output circuit. In modern applications, both power density (i.e., watts/volume) and power loss in the form of heat dissipation have become important and sometimes overriding factors. With increasing power density, more attention must be devoted to power losses and heat dissipation since, with the high electronic density found in modern computers and the like, the elmination of even relatively small avoidable power losses is an appreciable advantage. Minimization of the physical size of the power supply is similarly important and, of course, it is desireable that these objectives be obtainable at a reasonable cost.
One source of lost power in conventional switching power supplies is the output circuit, which includes an output transformer. This transformer has at least one primary winding receiving switched output current from a device, e.g., a high power switching transistor. Its secondary winding couples the switched power output to an output filter circuit that includes rectifying elements and a filter capacitor across which the output voltage is developed. Conventionally, the output circuit also includes an inductance, either as part of the output filter circuitry, or to supply current to the output filter capacitor during a segment of the switching cycle in which the switching device is open. The foregoing arrangement is found on both single-ended and balanced types of power supplies. In the former case, a single switching device is used, and switched current ordinarily is coupled to the output circuit through a single primary transformer winding and a single secondary winding. The latter type of supply implements at least two switching devices operating in alternation, and an output transformer which may take a number of configurations.
As is well understood in the art, the switched current is coupled through the output circuit transformer and converted by output rectifiers and the filter into a direct current output. Regulation of the output parameter, e.g., output voltage, is gained by controlling the duty cycle of switch operation. Control of the switched power duty cycle is obtained by continuously and automatically adjusting the duration of switch activation in accordance with incremental variations in the line and output voltages.
It has been the practice of the industry to maintain the output transformers and inductors as separate electrical components. Thus, the output transformer and the output inductor are kept physically and electrically independent. This independence has been continued even where, as in some prior art devices, the inductor includes a primary winding, in series with the transformer primary winding, poled to establish magnetic flux in the inductor core when the switching device is closed and to induce in a secondary winding a voltage of appropriate output polarity when the switching device is open.
The use of separate and completely independent magnetic components in the output circuit tends to increase manufacturing costs and add to the overall size of the power supply. We have found that the output transformer and the output inductor can be physically combined, and to a degree magnetically combined, so as to create a unitary structure which reduces manufacturing costs and provides an important savings in space on the power supply chassis. As explained in more detail below, the uniting of the output transformer and inductor includes the use of a common magnetic core segment which completes the magnetic flux paths resulting from exciting the transformer primary with switched output current. This common core segment also completes the path for flux established by current flowing through the inductor winding.
The common core segment is feasible, in part, because of the smaller operating excursions required in power supplies operating at high switching frequencies. This reduced amplitude of the flux excursions minimizes hysteresis and eddy current losses in the magnetics and also allows the common magnetic core section to accomodate both the inductor flux and the transformer flux without any significant increase in cross-sectional area of the core.
We have further discovered that the integrated magnetic output device, described briefly above, can advantageously incorporate an auxiliary winding for coupling leakage inductance energy from the transformer and/or inductor primary winding to the output.
In switching power supplies, some measures are required to avoid impressing excessive induced voltages across the switching device (usually a semiconductor) when it transfers from the closed state to the open state. If the switched current is suddently terminated, the voltage induced in the primary side of the transformer by the sudden collapse of current is capable of reaching enormous values. Unless something is done, the induced voltage can well exceed the breakdown voltage of the switching semiconductor. The practice in the art has been to shunt the primary winding of the transformer with some means of absorbing the leakage inductance energy in the transformer. Typically, such shunt includes a capacitor and a diode for establishing a current loop whereby the transformer primary current is used to charge the capacitor when the switch is open. When the switch again closes the energy stored on the capacitor is dissipated through a discharge path including the switch and a resistive element.
In accordance with the present invention, we have found that the leakage inductance energy from the transformer and associated wiring can be utilized to augment the power output capacity. This is achieved by transferring the stored capacitor energy (derived from the leakage inductance) to the inductor output winding (or to the transformer secondary) via an auxiliary winding magnetically coupled thereto. Current in the auxiliary winding sets up a flux in the magnetic core upon closure of the switch and when the switch opens, the flux induces a voltage that drives the output. Because this energy is supplied to the output circuit, it contributes to the power output demand and therefore increases the power supply efficiency.